Most pipelines must be assembled from a series of pipe sections abutted end-to-end, and many require that fluid, such as liquid chemicals or gases, or fluid-like material, such as fine particulate dust, be conveyed the length of the pipeline without leakage. In order to avoid leakage, the pipe section ends must be coupled in a manner which seals the joint. A coupling for joining two abutting pipe ends may typically include either two semi-cylindrical pieces fastened together to clamp down on the pipe ends, or a single cylindrical piece having an axial slit which allows the coupling to be opened by an amount sufficient to fit over the pipe ends.
Often, annular gasket members, such as O-rings, are placed around the pipe sections near their respective ends, to be compressed between the coupling and the outer surfaces of the pipe ends. The fluid pathway between the axial joints of the coupling then must be sealed. One approach, as shown in U.S. Pat. Nos. 2,913,262 and 3,153,550, has been to provide an axial gasket member extending between the O-rings within the axial joints of the coupling. A problem with this approach is that the coupling must be precisely closed about the pipe ends to effect a reliable seal. Another approach, as shown in U.S. Pat. No. 1,607,943 and in French Pat. No. 355,620, has been to provide an annular central web connecting the O-rings, completely surrounding the pipe ends and spanning the gap between the pipe ends. This solution has the disadvantage that if the web is physically pressed by the coupling against the pipe ends, sharp or ragged pipe ends may lead to early deterioration of the gasket. If the web is not pressed against the pipe ends by the coupling, as is the case in the above-identified French patent, then the pressure of fluid within the pipes must be relied upon to maintain the seal of the joint.
U.S. Pat. No. 4,360,227 discloses a successful pipe coupling and gasket for solving some of the problems in the art. The gasket includes O-rings joined by a web only in the area of the axial slit of the coupling. Assembly of the preferred embodiment requires molding of the shaped web and vulcanizing or otherwise bonding the web to the O-rings, plus the further attachment of a shield under the web, if desired. Installation on pipe ends requires that the pipe ends be held together while stretching both O-rings over the adjacent pipe ends, and while the coupling is fitted over the gasket and pipe ends.
Furthermore, the gasket material of the web is compressed against the pipe ends, which may move in some installations. Expansion and contraction of the pipe sections causes such movement. Also, pipelines positioned overhead or floating on a lake are not fixed or tied down, and the pipe joints may move considerably. In these installations, it is difficult to get the web and O-ring assembly to stay in place on the pipe ends during installation of the coupling.
In some such installations, one or both of the pipe ends are left smooth, without a ridge or flange to cause the coupling to pull the pipe ends firmly and rigidly together. The O-rings in such a case rest on the smooth surface of the pipe ends, and the pipe ends may shift, even to a position leaving a substantial gap between the pipe ends.
As noted above, the gasket shown in U.S. Pat. No. 4,360,227 may be provided with a metal shield between ragged ends of the pipe sections and the bottom of the web. Even with the shield in place, however, the web contacts the pipe sections and wear can result. Since this shield is positioned away from the intersection of the web and the O-rings, it may catch on the pipe ends during installation of the O-rings. Movement of the shield with the pipe ends is transmitted to the web, which is compressed against the shield. Such movement could dislodge the web from its proper sealing position. In a situation in which the pipe ends can separate to form a substantial gap, the shield would have to be made wider to avoid falling into the gap, but if allowed to approach the O-rings too closely, could interfere with the sealing function of the web or the O-rings. For these reasons the gasket of U.S. Pat. No. 4,360,227 is best used with couplings which positively lock the pipe ends together and minimize movement of the pipe ends.
Thus, there has been a need in the art for a means to seal the axial joints of pipe couplings that is easier to install and better adapted to the problems caused by movement of the pipe ends.